1. Field of the Invention
This invention relates to the manufacturing of vehicle suspension components, and more particularly, to a method and apparatus for producing linear grooves on the inner diameter wall of cylindrical tubes.
2. Description of the Related Art
A conventional damping device, such as a shock absorber or suspension strut, comprises a cylindrical pressure tube. Surrounding the pressure tube is a coaxially disposed reservoir tube. Located on the inner wall surface of the cylindrical pressure tube are linear grooves which define fluid flow paths. The grooves allow damping fluid to communicate between the two fluid chambers within the pressure tube defined by the opposite sides of a piston. Variations in the groove depth, width, length and a combination thereof, define the volumetric fluid flow around the piston, thereby developing the damping characteristics of the device. Proper damping characteristics mandate accurate and repeatable groove locational, dimensional and angular control during the forming process.
Methods previously utilized to produce linear grooves in the inner diameter of cylindrical tubes include machining and expanding tool devices. Grooves produced by machining operations tend to generate undesirable sharp-edged groove transitions and burrs. Groove profiles of variable sectional geometries require multiple machining operations and tool sets to produce. Accordingly, machining is not readily adaptable to high volume production environments.
The expanding tool method allows for radial expansion of a grooving anvil into contact with the inner diameter of a cylindrical tube. The expanded anvil is then forcibly pushed or pulled through the tube along its cylindrical axis. Movement of the unit generates flow of the pressure tube wall into the groove profile defined by the anvil tool. This process is not capable of producing grooves of variable cross-sectional geometries. Substantial forming pressures are also required.
A further method of forming a pressure groove is disclosed in U.S. Pat. No. 4,643,011. In this reference, the cylindrical pressure tube is introduced into a cylindrical support member. The support member contains predefined receiving grooves on its inner diameter. A roller member having a rolling axis perpendicular to the pressure tube cylindrical axis is introduced into the inner diameter of the pressure tube. The roller member has a radially projecting elevation that is angularly aligned with the receiving groove in the support member. As the roller member travels through the length of the pressure tube, the elevation is pressingly urging the pressure tube wall thickness into the mating support member receiving groove. The grooves are formed by embossing the pressure tube wall thickness into the receiving grooves of the external support member. This method often requires special support member for different groove profiles. Additionally, simultaneous production of multiple grooves circumferentially located within the pressure tube is generally not possible. Further, significant forming pressures are often required by this method.